1. Field of the Invention
The present invention relates to subsoil exploration from seismic data.
The invention concerns seismic data processing allowing providing of a high-resolution image of the subsoil. Such an image is used in the sphere of petroleum exploration and production, or of geologic CO2 storage site monitoring.
2. Description of the Prior Art
In the petroleum industry and for CO2 storage site monitoring, it is very important to have precise images of the subsoil. Precision is defined in terms of resolution. The higher the resolution, the more data on the structure or the composition of the subsoil in the image. It is desired to construct, “high-resolution” images of the subsoil, whether considered to be static (case of petroleum exploration) or dynamic (case of underground reservoir monitoring).
Measurements are performed within the subsoil in order to establish an image thereof. Two main types of measurement are conventionally used: measurements within wells drilled through the formation and seismic surveys. The first ones allow perfectly defining the properties of the subsoil, either through coring or through logging (continuous measurement of various properties of the subsoil along the well), then interpretations of the logs are thus obtained. The information is precise but highly localized around the well. On the other hand, seismic data play a role by imaging a large volume of the subsoil, but the information is less precise and the resolution lower.
However, many techniques have been developed to improve the precision of the information obtained from seismic data. In particular, seismic quantitative imaging methods intended for estimation of the distribution of certain parameters in the subsoil, such as the acoustic impedance or parameters linked with the impedance, represent a significant advance in comparison with the way a conventional seismic image is obtained. Of course, the quality of the result provided by these methods is all the higher as the resolution of the image directly resulting from seismic data is high.
Between these two main types of measurement, there are seismic well data acquisition methods. They emit seismic waves in the subsoil and record the response (notably reflections) of the subsoil with receivers arranged in a well.
One example of such well survey techniques is the “walkaway” type acquisition method. This technique is for example described in:    Mari J.-L., Glangeaud F., Coppens F., 1997, “Traitement du Signal Pour Géologues et Géophysiciens”, ÉditionsTechnip.
As illustrated in FIG. 1, a walkaway acquires seismic data by arranging seismic sources (S1, S2, . . . , Sp) at the surface, generally in a rectilinear layout through the well, and seismic receivers (R1, R2, . . . , Rn) in the well, at various depths. In this figure, x represents a geographic direction and z represents depth.
FIG. 4 shows typical data obtained by this type of acquisition. These data show a downgoing wave train having many arrivals: the first arrival does not really dominate the following ones (referred to as second arrivals). This observation provides the guiding line of the standard seismic well data processing: it is based on a separation of the data into upgoing waves and downgoing waves, followed by a deconvolution of the first by the second.
This approach however rests on a 1D view of the propagation of waves, which is not really realistic, especially for data where the source is not vertical to the well.
As soon as the wave propagation occurs in directions other than the vertical, an estimation of the velocity distribution is essential for processing. In the event that the earth is considered to be a 1D medium, extensions to the processing presented above have been proposed which are based on the application of dynamic corrections (offset correction or NMO).
This concept does however not account for the seismic events generated by the second arrivals. In fact, the correction to be applied to these events is in no way intrinsic and it depends on the event. If there were no second arrivals and, more generally, no multiple reflections, which is not a really realistic hypothesis, seismic well data imaging could be achieved by a simple migration of the records, and the velocity distribution would have been estimated beforehand. If certain precautions have been taken, an estimation of the subsoil reflectivity (quantity linked with the impedance) would thus be obtained. However, due to the particular illumination of the medium (linked with an acquisition device located in a well), the result contains distortions, as illustrated in FIG. 1, whereas the desired subsoil model, illustrated in FIG. 3, is practically a 1D model.
The following document presents known techniques for seismic well data processing: Bob A. Hardage, Collection Handbook of Geophysical Exploration, Geophysical Press, 1985.
Thus, the prior art techniques have difficulty reconciling accounting for multiple reflections with multi-dimensional propagation and, all the more, in media other than 1D media. Besides, these imaging techniques use linear processes (deconvolution or migration), which consequently limits the resolution of seismic images to the seismic frequency band (of the order of λ/2 if λ is the seismic wavelength).
The present invention is a method of constructing an image of the subsoil by accounting for the multiple reflections as well as the multi-dimensional character of the propagation. Furthermore, through the use of a non-linear imaging technique, the invention substantially improves the vertical resolution and notably goes well beyond the seismic frequency band (from λ/2 to λ/10 depending on the heterogeneity of the sediments).
Moreover, the invention relates to a method of constructing an image representative of a distribution of acoustic impedances in an area of the subsoil, by seismic measurements acquired with a configuration comprising an emission of seismic waves from the surface into the subsoil and the reception thereof by receivers positioned at different depths in at least one well. The method also requires an estimation of the seismic wave propagation velocity field in the subsoil. The method comprises the following:
selecting p illumination angles, each illumination angle corresponding to a direction of propagation of a wave front at the level of an upper limit of an area;
organizing seismic measurements into data Dp organized by illumination angle p;
determining, within the area, an acoustic impedance distribution by an inversion, wherein a difference between the data Dp obtained from the seismic measurements and data resulting from an estimation is minimized by an equation of wave propagation from a velocity field, an acoustic impedance distribution and a pressure distribution at the level of said upper limit of the area for each illumination angle.
In order to minimize the difference, a priori information defined by a direction of a dip concerning a structuration of the acoustic impedance distribution, and a direction of a dip concerning a structuration of the pressure distribution can be taken into account. It is therefore possible to minimize a least-squares function comprising: a first term measuring the difference, a second term corresponding to a directional derivative concerning the structuration of the acoustic impedance distribution and weighted by a weight εI, and a third term corresponding to a directional derivative concerning the structuration of the pressure distribution and weighted by a weight εB,p. The weights can be determined by a trial-and-error technique. The following rules can also be applied:    a result showing too strong correlated residues express too strong a value of at least one of the two types of weight; and    a result showing an acoustic impedance distribution with abnormally weak lateral variations which expresses too great a value assigned to weight εI.
According to the invention, a term can be added to the function to take into account a priori information determined at the level of at least one well drilled through the area.
In order to organize the seismic data into data by illumination angle, a Radon transform can be used.
According to the invention, the image can be interpreted in lithologic and/or petrophysical terms, so as to monitor a geologic acid gas storage site or to locate and assess oil reservoirs. The image can also be used as a complement to other logs so as to characterize the area.